The invention disclosed herein relates to a transport system for sheet media, particularly for advancing a sheet medium past a print or plot head in a printer or plotter.
Printers and plotters of the contact type (e.g., pin dot matrix, thermal transfer, direct thermal and pen plotter) employ print or plot heads which contact a sheet medium in order to form an image on the sheet medium, as opposed to non-contact types which typically use electrophotographic imaging techniques. Contact printers and plotters in which the sheet medium is moved past the contact-type print head during printing introduce drag on the drive system which moves the sheet medium. Such drag may present a problem if it is excessive or localized, as discussed below. Typically the sheet medium is moved relative to the print or plot head along the length of the sheet medium, which by convention is parallel to the X-axis of an X-Y coordinate system, and a so-called flying or serial print head is moved parallel to direction of the sheet medium. The drive system which moves the sheet medium is typically referred to as the X-axis drive system.
Such X-axis drag does not typically present a problem when it is distributed across the Y-axis, as for example in line mode type printers where the print head is the same width as the sheet medium. Also, such X-axis drag does not occur where the print head is moved parallel to the Y-axis relative to a stationary sheet medium while printing a line of the image, only after which is the sheet medium moved in the X-direction, as in serial or raster scan-type printers.
Although localized X-axis frictional drag is generated by the down-force of a pen-type plot head against the sheet medium and the supporting platen as the sheet medium is moved past the plot head, it does not usually present a problem because the force is relatively low, e.g., only in the order of 30 grams, which is generally insufficient to cause misregistration of the sheet medium with the X-axis drive system.
In the novel strip mode printing disclosed in application Ser. No. 920,186, printing proceeds in X-axis strips, strip by strip, i.e., a strip which is only part of the Y-axis width of the sheet medium is printed with the print head of the printer held stationary in the strip, then the print head is moved or indexed in the width direction (Y-axis) of the sheet medium to another strip which is printed while the print head is held stationary in that strip. Each strip is substantially narrower than the full width of the sheet medium to be printed upon, and typically several X-axis strips are required to print across the full width of the sheet medium. In thermal transfer strip mode printing as described in Ser. No. 920,186, a receptor sheet medium is continuously moved by the X-axis drive system while the stationary thermal print head of the thermal transfer printer prints the particular strip. The thermal printer utilizes a thermal transfer or donor medium containing a heat activated pigment, wax, resin, ink, etc. layer thereon (hereinafter referred to as "ink") which is transferred in a desired pattern to the receptor medium by the thermal print head. The receptor sheet medium may be paper, plastic, mylar, etc., and the thermal transfer ribbon may be any conventional film having a heat-activated "ink" layer thereon.
During thermal transfer printing, thermal elements in the print head contact the thermal transfer ribbon and press the ribbon against the sheet medium which is supported by a platen. By heat and some pressure the print head activates and transfers the ink carried by the ribbon onto the sheet medium. The ribbon and the sheet medium are maintained in contact and heat is applied by the print head for a predetermined minimum "dwell" time sufficient to effect transfer of the ink to the receptor sheet medium. Typically, the thermal transfer ribbon becomes temporarily adhered to the sheet medium during the dwell time as the ink layer is melted and the ink transferred to the sheet medium. The sheet medium is typically continuously moved past the print head at a rate slow enough to permit the print head to heat and press the ribbon against the sheet medium for at least the minimum required dwell time. The combination of the tension on the ribbon and the down force of the print head against the ribbon and the sheet medium while the ink layer on the ribbon is being melted and the ink transferred to the sheet medium create a drag in the X-axis on the continuously moving sheet medium.
The thermal print head in the thermal printer disclosed in application Ser. No. 920,186 is longer (Y-axis direction) than a typical serial print head and substantially shorter than the width of the sheet medium, e.g., up to about four inches (10.16 cm), so that localized X-axis drag forces occur when the print head is pressed against the sheet medium as the sheet medium is moved in the X-axis relative to the print head. For example, in the thermal printer of application Ser. No. 920,186 the down-force of the thermal print head and transfer ribbon during heating thereof against the sheet medium is typically 1000 grams or more. Such a large down-force produces a resultant frictional drag force, e.g., 525 grams, and when this is applied off center of the sheet medium, sufficient distortion of the sheet medium may occur to cause misregistration of the sheet medium with the X-axis drive system.
Although this drag problem and its solution are described herein in connection with thermal transfer printers, the invention is applicable to other types of printers and plotters where the print head introduces localized X-axis drag against the sheet medium as it moved past the print head, and to other apparatus which transports sheet media in which the sheet medium is subjected to the types of frictional problems described herein.